This invention relates to the control of bearing load in assemblies incorporating shafts mounted in bearings. The invention is especially, but by no means exclusively, applicable in relation to compressor and turbine shaft bearings in gas turbine engines.
Mechanical bearings incorporating bearing elements such as balls or rollers contained in bearing races have optimum performance and hence minimum wear when operating under the loading conditions for which they are designed. At loads other than the optimum loading, and particularly at loads approaching zero, slippage of the bearing races can occur. This in turn can give rise to overheating and/or damage to components of the bearing resulting in reduced bearing life.
Gas turbine aircraft engines are generally mounted on aircraft at a small inclination to the horizontal. When the aircraft is at rest there is therefore a loading in one direction on the bearings in which the main compressor and turbine shafts are mounted. When the engine starts, air is drawn into the engine and loads the shaft bearings in the opposite direction. As a result the bearings pass through a zero load condition during start up. This zero load point can also be reached during transient manoeuvres. It is an object of the present invention to provide a method and means whereby this condition is maintained for as short a time as possible.
According to one aspect of the invention there is provided a method of controlling bearing load comprising sensing the approach of a no-load condition in a bearing, generating a signal in response thereto, and applying a load to the bearing in an appropriate direction in response to said signal.
Preferably the approach of the no-load condition is sensed by sensing the relative speed of a bearing race and rolling elements of the bearing. We have found that such changes in relative speed result in changes in electrostatic charges at opposite sides of the rolling elements of a bearing, hence signalling the approach of a no-load condition. Accordingly said sensing is preferably effected by sensing changes in electrostatic charges in the bearing.
Thus according to a further aspect of the invention there is provided a method of sensing the approach of a no-load condition in a bearing comprising sensing changes in electrostatic charges in the bearing generated on the approach of the said condition. Advantageously the method may be employed in a method of controlling bearing load by generating a signal in response to said changes and applying a load to the bearing in an appropriate direction in response to said signal.
The direction of said applied load may correspond to the direction of loading of the bearing before the approach to said no-load condition to restore the original loading condition. Alternatively, the direction of the applied load may be opposite to the direction of loading before the approach of said no-load condition, whereby to move the bearing quickly through the no-load condition to a condition in which it is loaded in the opposite direction.
Preferably said applied load is produced by an electromagnetic actuator operated in response to said signal.
The invention also provides apparatus for controlling bearing load comprising sensing means for sensing the approach of a no-load condition in the bearing, means for generating a signal in response thereto, and means for applying a load to the bearing in an appropriate direction in response to said signal.
Preferably, said sensing means comprises means for sensing changes in the relative speed of a bearing race and rolling elements of the bearing. Advantageously said sensing means comprises means for sensing changes in electrostatic charges in the bearing resulting from said changes in relative speed.
Said sensing means is preferably a non-contact sensor. The sensing means may comprise a proximity sensor, a radioactive tracer or, preferably, an electrostatic sensor. The invention also provides apparatus for sensing the approach of a no-load condition in a bearing including means for sensing changes in electrostatic charges in the bearing generated on the approach of the said condition. Advantageously the apparatus may include means for generating a signal in response to said changes and means for applying a load to the bearing in response to said signal.
The direction of said applied load may correspond to the direction of loading of the bearing before the approach to said no-load condition to restore the original loading condition. Alternatively, the direction of the applied load may be opposite to the direction of loading before the approach of said no-load condition whereby to take the bearing quickly through the no-load condition to a condition in which it is loaded in the opposite direction.
Preferably said means for applying said load comprises an electromagnetic actuator. Preferably said actuator comprises a variable or xe2x80x98activexe2x80x99 electromagnetic bearing surrounding said shaft. Preferably said electromagnetic bearing has both radial and axial thrust capabilities.
As applied to the main shaft bearings of a gas turbine engine, said load is preferably applied in the opposite direction to the initial direction of loading, whereby to take the shaft bearings through the no-load condition generated on start-up in a rapid and controlled manner.
The invention also includes a gas turbine engine including apparatus for controlling bearing load as aforesaid.